The present invention relates to power control systems for motor vehicles, and more particularly to such control systems for switching a vehicle alternator to a "high-voltage" mode to power a high-voltage accessory.
With increasing regularity, motor vehicles include accessories which require abnormally high voltages (i.e., in excess of 12 volts) for their operation. Such accessories include resistively heated windshields and windows, and resistively heated elements for heating the passenger compartment. Accordingly, a wide variety of power control systems has been developed for selectively operating the vehicle alternator to output an abnormally high voltage. Examples of such systems are shown in U.S. Pat. No. 4,074,145, issued Feb. 14, 1978, to Laffoon et al, entitled ELECTRIC POWER GENERATING APPARATUS; U.S. Pat. No. 3,676,694, issued July 11, 1972, to Schneider et al, entitled POWER OUTPUT ACCESSORY UNIT; and U.S. Pat. No. 4,267,433, issued May 12, 1981, to Sahm, entitled ELECTRICALLY HEATABLE WINDSHIELD HEATER CONTROL CIRCUIT.
Both the Laffoon and Schneider control systems are switchable between "standard" and "high-voltage" modes. In the standard mode, the field winding of the vehicle alternator receives an excitation voltage from the voltage regulator in conventional fashion to charge the battery and power the distribution system. In the high-voltage mode, the field winding receives an excitation voltage from the battery through a potentiometer or variable resistor to power only the high-voltage accessory. The potentiometer is manually adjustable, enabling the level of the higher alternator output voltage to be selected for a given engine speed. However, the high voltage varies with engine speed regardless of the potentiometer setting. Consequently, the Laffoon and Schneider control systems provide a varying voltage in the high-voltage mode. This is unacceptable in many commercial applications, wherein proper operation of the accessory is highly dependent on a uniform voltage level.
In the Sahm power control system, the high-voltage accessory--namely a heated windshield--can be selectively coupled in series between the alternator and the battery. A single voltage regulator assembly including a voltage divider maintains a constant standard voltage at the battery regardless of the voltage applied to the windshield. As with the Laffoon and Schneider power control systems, the high voltage applied to the windshield in the high-voltage mode, varies with engine speed. As pointed out above, this is unacceptable in many applications.